1. Field of the Invention
This invention relates to a semiconductor device and more particularly to a contact portion in a semiconductor device.
2. Description of the Prior Art
Referring to FIG. 1, a structure of a conventional contact portion in a semiconductor device is schematically illustrated in a fragmentary sectional view. A method for fabricating this conventional contact portion is as follows. First, an insulator layer 2 is formed on a main surface of a silicon substrate 1 by thermal oxidation, CVD, sputtering or the like, and then a contact hole 8a is opened through the insulator layer 2 by photolithography. Subsequently, a doped layer 4 is formed by ion implantation and thermal diffusion or the like, and then a metal film 3 is deposited by vacuum evaporation, sputtering, CVD or the like. Finally, the metal film 3 is patterned to complete lead wires.
FIG. 2 shows another conventional contact portion which has a contact hole 8b smaller than that of FIG. 1.
FIG. 3 shows a further contact portion according to the prior art, in which a contact hole 8c is tapered by adjusting etching conditions during anisotropic etching.
FIG. 4 shows a still further contact portion according to the prior art, in which a contact hole 8d comprises a first portion 8d.sub.1 formed by isotropic etching and a second portion 8d.sub.2 formed by anisotropic etching.
When the aspect ratio (proportion of the depth to the width) of a contact hole is as small as about 0.5, as in FIG. 1, relatively good coverage of a metal film 3 can be obtained in the contact hole. In the case that Al-Si is deposited as a metal film 3 by sputtering, the thickness of the metal film in the hole reaches 30-40% of that on the top surface of the insulator layer 2. However, when the aspect ratio is about 1.0 as in FIG. 2, the thickness of the metal film in the hole decreases to less than 10% of that on the insulator layer. Accordingly, it is likely that disconnection will occur in the metal wire. In view of this problem, the coverage of the metal film in the hole has been improved in the prior art by providing a taper in the hole as shown in FIG. 3 or by utilizing two-step etching as shown in FIG. 4.
Since a contact portion in a prior art semiconductor device is structured as described above, there exist the following problems.
(I) The contact hole is not filled up, though a flat top surface of the substrate is desired in the technical field of the multilevel interconnection. PA0 (II) The improved contact holes as shown in FIGS. 3 and 4 are relatively large in size and thus are not preferred in view of miniaturization of a semiconductor device.